1. Field of the Invention
The present invention relates to a cogeneration system, and particularly to a cogeneration system for on-site generation of electrical and thermal energy.
2. Description of the Prior Art
Cogeneration to supply on-site electrical and thermal energy needs is a proven technique which is becoming cost effective for smaller energy users as its efficiencies of operation improve. Federal legislation also provides significant economic incentives for smaller users to turn to cogeneration. Utilities are required to provide back-up power on shut down of a cogeneration unit and must purchase any excess electricity generated.
There has been considerable research and development effort in the prior art to develop an economically viable cogeneration unit for the smaller energy user with both power and thermal energy needs. A satisfactory unit must be compact and capable of fully automated, substantially noise-free operation. It has to be simple to install and operate and, above all, it must be highly efficient in converting a high percentage of the energy in the operating fuel into electrical and thermal energy for all on-site electrical, cooling and heating needs, at a price competitive with central utility services.
A typical prior art cogeneration unit employs a water cooled internal combustion engine in combination with an electrical generator. The efficiency of such an engine generator combination depends to a great extent upon the amount of so-called "waste" heat which can be recovered from the engine exhaust and engine coolant for heating and cooling needs. In many instances the engine-generator set is mounted in the open on a concrete pad or the like and no effort is made to recover heat which is lost through radiation to the atmosphere. In fact, many designs rely on heat radiation for engine cooling.
In some systems of the prior art the engine and generator are housed within a thermally insulated enclosure to capture radiated heat, and also to attenuate the sound level of operation. U.S. Pat. Nos. 4,262,209, issued Apr. 14, 1981 to Berner, and 4,495,901, issued Jan. 29, 1985 to Nannini et al, are representative of this type of arrangement. The latter patent teaches a system in which intake air for the engine is circulated through the enclosure for preheating, which tends to capture some of the radiated heat and reduce the air temperature in the enclosure. However, there is evidence that the carburetor is incapable of drawing in enough heated air to maintain an enclosure temperature sufficiently low to prevent heat damage to various externally located engine and generator components. In addition, preheating of the air results in a less dense fuel charge to the engine and undesirably reduces the rated horsepower of the engine.
U.S. Pat. Nos. 4,226,214, issued Oct. 7, 1980 to Palazzetti, and 4,548,164, issued Oct. 22, 1985 to Ylonen et al also disclose cogeneration units housed in insulated enclosures through which air is circulated.
An added disadvantage of prior art systems employing an engine enclosure is their location of the carburetor in the enclosure. This presents a serious explosion hazard because there is always the possibility of leakage of the fuel/air mixture into the enclosure.
Without exception, prior art systems capture radiated heat inefficiently, if at all. The air circulation systems have the disadvantages mentioned, and also necessarily require a circulation fan or heat exchanger or the like, all of which add expense and complexity.
Yet other systems of the prior art provide a combination of sound attenuation and engine cooling by placing the engine within a watertight enclosure which in turn is submerged in a tank of water. Such systems are disclosed in U.S. Pat. Nos. 3,723,027, issued Mar. 27, 1973 to Montellus, and 3,805,082, issued Apr. 16, 1974 to Murray. The system of the latter patent circulates air through the enclosed engine space for engine cooling.
None of the prior art teachings appear to recognize the importance of preventing radiation heat loss. When the engine is enclosed in a thermally insulated enclosure, heat is radiated until the enclosure air reaches a temperature approximating that of the engine. On engine start-up the enclosure acts as a heat sink soaking up heat and slowing engine warm-up, and then losing that heat on engine shut-down. Frequent engine start-ups and shut-downs significantly reduce the efficiency of the system. The situation is not greatly improved if a circulating air fan is used to scavenge some of the heated air for use as engine intake air, as discussed above, and heat exchangers are not sufficiently efficient. It seems apparent that a loss of radiated heat is accepted in the prior art as a necessary consequence of engine cooling.